Crack Azimuth Sequences on Europa: the Southern Leading Hemisphere

Introduction: Galileo images of Europa's northern hemisphere taken at low resolution during the first orbit of Jupiter showed that regional-scale lineaments' orientations followed a systematic progression (clockwise going forward in time) consistent with predicted trends in tidal stress over ~1/6 of a period of non-synchronous rotation [1]. The fit is even better if diur-nal tides, as well as non-synchronous effects, are taken into account [2]. Non-synchronous rotation, i.e. a gradual rotation relative to the direction of Jupiter, had been predicted [3], based on considerations of tidal torques caused by Europa's orbital eccentricity. The possibility that tectonic patterns might contain a record of non-synchronous rotation was noted by [4] and [5]. Other lines of evidence based on Galileo imaging corroborated the likelihood of nonsynchronous rotation , including studies of tidally driven strike-slip tec-tonics [6] and of the formation sites of cycloidal cracks under tidal stress [7]. The difference from synchronous rotation is small (the period relative to Jupiter is >12000 yr) according to a comparison of Voyager and Galileo images [8]. The evidence regarding Europa's rotation is reviewed in [9]. Geological mapping of other regions did not provide such compelling evidence of non-synchronous rotation. In the northern leading hemisphere, an azi-muth sequence inferred from geological mapping, when compared with the tidal stress model from [2], required >2 non-synchronous rotation periods [10]. In high-resolution images of the densely ridged terrain north of Conamara, the sequence of crack azimuths seems to require >3 non-synchronous rotation periods [11]. Neither of those studies found a continuous (many cracks over a small fraction of a period), monotonic variation in azimuths that would strongly support the non-synchronous model. Two other studies also raise questions about the simple relationship between Europa's rotation and monotonic change in the crack azimuth. First, in the Astypalaea region (the far south in the anti-Jovian hemisphere) the order in which the most recent cycloi-dal ridges formed does not match the sequence of longitudes at which they likely formed according to tidal stress theory, suggesting that only one or two cracks form in a given region per rotation period [12]. Second , a study of strike slip faults found evidence for significant polar wander, fairly recent in the age of Europa's surface [13]. A preliminary study of the southern leading hemisphere identified three sets of lineaments, each set